Direct Digital Synthesizer Signal Generator
I have built many analog oscillators which work fine over an RF frequency range of about 2:1 (eg 5 MHz to 10 MHz), and are as accurate and stable as the components. My oscillators in the Electroluminescent Receiver wander a few tens of Hertz per minute. With advances in digital technology Direct Digital Synthesizers (DDS) have been available for a few years and are now replacing Phase-Locked Loops in some professional receivers. The signal generator shown above is built using an AD9851 (10-bit resolution) DDS kit from minikits, with continuous frequency coverage from 0 - 65 MHz, resolution and stability of 1 Hertz. The output of the kit uses a slightly overdriven MAR6 MMIC for about +7 dbm output, which I increased using a THS3202 current-feedback Op Amp (see circuit below) for additional output.
Actually, the DDS does not cover the range 0 - 65 MHz. There is a low frequency roll-off (the - 3 dB frequency s about 800 kHz in my circuit above), which can be descreased a little by increasing the values of the coupling capacitors, but the kit is intended for RF usage rather than the audio range. I have set the lower limit to 450 kHz which allows me to test 455 kHz IF strips. There is also small passband ripple (around 1 dB) between about 47 MHz and 65 MHz due to the LPF at the output of the AD9851 before the MAR6.
The THS3202 is an excellent wideband Op Amp. As there are two amplifiers in one SOIC package I used one to boost the signal and the other as a buffer amplifier with a gain of about 1 (1 + 560/2700 = 1.2). The THS3202 will drive 20 Ohm loads, and on a single 12 Volt supply the large signal voltage swing is from about 2 V to 10 Volts (ie about 2.8 Vrms or about +22 dBm). The 10 Ohm resistor is to assist driving capacitive loads. Selection of Rf feedback resistor is important for widest bandwidth but is less important in the circuit above where operation is only required to 65 MHz. The THS3202 has a flat response to nearly 1 GHz. The 1 kOhm resistors provide mid-supply bias, and the input resistance can be set by Rin (I have used 470 Ohm). Working with SOIC packages requires tweezers and patience. (The photo of the signal generator internals above was taken before the amplifier board was added).
The THS3202 is also suitable for RF and IF usage. It could be used as a buffer amplifier for driving diode ring mixers (50 Ohm, +17 dbm) with very few additional components. Output intercept point with a gain of 5 is in excess of +36 dBm at 50 MHz, so could successfully be used as RF or post-mixer amplifiers without inductors.
I have tried the DDS as the oscillator for the Electroluminescent Receiver, and whilst it does work very well there are spurios at various positions across the bands resulting in 1 kHz whistles. As a signal generator the DDS is stable, highly accurate, can quickly be set (with keypad or rotary encoder), but is not absolutely spectrally pure over the entire frequency range. The AD9851 uses 10-bit encoding of the sine wave, whereas the newer AD9854 is 12-bit and should be better.
Current drain is just under 40 mA (about 20 mA per amplifier). The DDS is also fairly current hungry. With the 60 mA drain of the LCD backlight, the total current drain is around 300 mA at 12 Volts, hence the generous heatsink on the back of the case. The AD9851 runs quite warm in operation (and I've heard that the AD9854 will overheat if not properly heatsunk).
Refer to the minikits page for details of the AD9851 kits. I have the works, with EME85, EME129 control board (only 3 switches loaded - Step, Cal and Memory), 30 MHz oscillator, 70 MHz LPF, Keypad, LCD and connector kit. THS3202D can be obtained from Digikey for about US$6.
© In the Light, 17 September, 2009 , Disclaimer, Son of Suckerfish drop-downs from HTML dog